Physics: Friction and Gravity

The strength of the force of friction depends on two factors: the types of surfaces involved and how hard the surfaces push together.

There are different kinds of friction:

• sliding friction - when one solid surface slides over another.
• rolling friction - when a object rolls over a surface.
• fluid friction - when an object moves through a fluid. 

Gravity

Free fall - when the only force acting on a falling object is gravity.

Terminal velocity -  the maximum velocity a falling object achieves.